I've had a brief look/think for a solution from South Africa, I think it would be worth your while to sign-up for a voipstunt.com prepaid account.
Depending on your set-up, this account allows calls in many ways; flexible e.g. has (local rate) 0105002389 Johannesburg access number.
Rates are pretty competitive to India landline & Mobile at £0.006/min or R0.116/min. I'm sure the local telcos would charge a lot more than that!
http://www.voipstunt.com----------------------------------------------
If you don't mind slight inconvenience/reduction in voice quality, you could potentially bring the cost down to zero using the free minutes offer to a UK 03xx if allows, using (03xx to 0870 callthrough) dialling the 0870 access number I have listed in the forum to call India. (Much of this working will depend on any UK service used accepting your international call).